Geographic Range

Western pipistrelles, Pipistrellus hesperus, occur from southern Washington to the southern portion of Mexico including the Baja peninsula. Stretching from the west coast of California, its range includes Arizona and New Mexico, spreading into western Texas and extreme western Oklahoma (Harris, 2001). These bats are also found as far north as Utah and southern Colorado in the eastern portions of their range (Klingel, 2000)

Habitat

Western pipistrelles inhabit a variety of habitats, ranging from rocky canyons, cliffs, and outcroppings to creosote bush flats. They are the most abundant of North American bats that are found in deserts, but are found at higher elevations in arid brush lands, grasslands, and even some forests. Western pipistrelles spend their days roosting in rock crevices, beneath rocks, in burrows, mines, and buildings. It has been suggested that western pipistrelles use burrows made by kangaroo rats (Dipodomys spp.) and other rodents (Barbour and Davis, 1969).

Western pipistrelles spend their winter hibernating in mines, caves, and rock crevices.

Water is a very important resource determining the distribution of this bat. Because of the high proportion of protein in their diet, the arid environment that they inhabit, and the subsequent high levels of evaporative water loss, western pipistrelles generally roost close to a water source.

Availability of maternity roost sites is an extremely important factor in successful bat reproduction (Cockrum and Cross, 1964).

Physical Description

Western pipistrelles are small brown-gray bats with distinct black, leathery facial mask, ears, and patagium. Beneath, they are pale brown-gray. Western pipistrelles have a short, blunt, and slightly curved tragus. They have a dental formula of I 2/3, C 1/1, P 2/2, M 3/3 = 34.

The species is sexually dimorphic, with females being larger than males. Weight ranges from 3 to 6 grams. Males have a total length of 66 mm, tail length of 27 mm, foot length of 5 mm, forearm length of 28 mm, and a wingspan of 19 cm. Females have a total length of 73 mm, a tail length of 30 mm, a foot length of 5 mm, a forearm length of 28 mm, and a wingspan of 22 cm (Davis and Schmidly, 1994).

Development

See Reproduction.

Reproduction

The mating behavior of these bats has not been characterized.

Copulation and insemination first occur in the fall. Sperm may be retained within the female through the winter. If aroused from hibernation, the female may empty the sperm from the reproductive tract. However, males continue to produce sperm during the winter, and winter copulations may be essential to guarantee spring fertilization. Pregnant females are found in May and June (Harris, 2001).

Young are born in June and July, after a 40 day gestation period. There are usually two offspring, but sometimes females produce only one. Newborn bats weigh less than 1 gram, but grow quickly. They begin to fly and are hard to distinguish from adults by about 1 month of age (Harris, 2001). Weaning occurs around one month of age, when the young are difficult to distinguish from adults.

The young are altricial, but grow and develop quickly, becoming practically indistinguishable from adults by 1 month of age. Females care for the young and provide them with milk. Sometimes young are reared by a female who lives along, but more commonly they are reared in large maternal colonies (Harris, 2001).

Lifespan/Longevity

Longevity of these bats has not been reported.

Behavior

Western pipistrelles begin their foraging flights very early in the evening hours, making them one of the most diurnal of North American bats. They may also be encountered later in the evening (4 hrs after sunset), or closer to the morning. Occasionally, individual bats have been observed on wing during mid-day, during which time they seek out water to alleviate stress caused by the arid environment they inhabit. Because these bats fly slowly, they are restricted to small foraging circuits. They have a very slow, fluttery flight that can often be observed along cliff faces, among pinyon trees, or other desert shrubs. They are often mistaken for large moths.

One of the smallest and weakest of all bats, a slight breeze can bring western pipistrelles to a standstill, and a stronger wind will cause them to seek shelter. In the winter, western pipistrelles will enter hibernation in the northern portions of their range, but also use hypothermia to conserve energy during periods of cold temperatures in the southern portion of their range (Cockrum and Cross, 1964; Cox, 1965). Males will exhibit more activity during these periods of reduced activity then females will.

Communication and Perception

Food Habits

Western pipistrelles are insectivorous bats that feed on a variety of insects depending on the time of year. Prey items are located using echolocation. Stomach content analysis has shown that these bats typically feed on one type of insect at any one time. This suggests that they will find a swarm of some type of insect and feed upon it exclusively. Western pipistrelles forage from 2- 15 m above ground on small, swarming insects and consume about 20 % of their weight in insects per night (Klingel, 2000).

Predation

Ecosystem Roles

Due to their high-energy requirements, western pipstrelles have a substantial impact on insect populations, potentially affecting a variety of insect-related ecological processes such as herbivory, pollination, and disease transmission (Klingel, 2000).

Economic Importance for Humans: Positive

Bat guano is an important source of fertilizer and houses unique organisms that are useful in the production of waste detoxifying agents (Snow et al., 1993). Because this bat preys heavily on insects, it also helps to control insect pests.

Positive Impacts

produces fertilizer

controls pest population

Economic Importance for Humans: Negative

Bats are known hosts of the causative virus for rabies and are known vectors of other zoonotic diseases.

Conservation Status

Although this species is common, and currently has no special conservation status, people should remain mindful that their activities could greatly reduce the western pipistrelle population, if steps are not taken to ensure that bat habitat is protected.

Like all bats, western pipstrelles are most vulnerable at their roost sites and therefore protection of these roost sites is the focus of most management. Since reopening abandoned or inactive mines can negatively affect bat colonies, it has been recommended that multi-seasonal surveys be conducted prior to allowing any renewed mining. These same issues should be considered when a mine is recommended for permanent closure. If bats are roosting in a mine scheduled for closure, installation of a bat gate is preferred method for preventing unauthorized human access while allowing the bats passage.

Human disturbance can be extremely detrimental to bat colonies, especially to non-volant young and hibernating adults, depending on the season and severity of the disturbance. Mines that are in close proximity to roads, towns, hiking trails or camp grounds are more susceptible to disturbance then those in remote areas with difficult access.

There is a need for greater assurance that roosts will remain undisturbed and that future (potential) roost sites will be left when managing for bats in pinyon-juniper habitat. Ideally, management should aim to sustain adequate food, water, and roost sites in close proximity to one another (Klingel, 2000).

Temperate North American bats are now threatened by a fungal disease called “white-nose syndrome.” This disease has devastated eastern North American bat populations at hibernation sites since 2007. The fungus, Geomyces destructans, grows best in cold, humid conditions that are typical of many bat hibernacula. The fungus grows on, and in some cases invades, the bodies of hibernating bats and seems to result in disturbance from hibernation, causing a debilitating loss of important metabolic resources and mass deaths. Mortality rates at some hibernation sites have been as high as 90%. While there are currently no reports of Parastrellus hesperus mortalities as a result of white-nose syndrome, the disease continues to expand its range in North America. (Cryan, 2010; National Park Service, Wildlife Health Center, 2010)

Contributors

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

chemical

uses smells or other chemicals to communicate

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

crepuscular

active at dawn and dusk

delayed fertilization

a substantial delay (longer than the minimum time required for sperm to travel to the egg) takes place between copulation and fertilization, used to describe female sperm storage.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

insectivore

An animal that eats mainly insects or spiders.

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

sperm-storing

mature spermatozoa are stored by females following copulation. Male sperm storage also occurs, as sperm are retained in the male epididymes (in mammals) for a period that can, in some cases, extend over several weeks or more, but here we use the term to refer only to sperm storage by females.

suburban

living in residential areas on the outskirts of large cities or towns.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

urban

living in cities and large towns, landscapes dominated by human structures and activity.

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

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The Animal Diversity Web is an educational resource written largely by and for college students. ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe. Though we edit our accounts for accuracy, we cannot guarantee all information in those accounts. While ADW staff and contributors provide references to books and websites that we believe are reputable, we cannot necessarily endorse the contents of references beyond our control.

This material is based upon work supported by the
National Science Foundation
Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.